This invention relates to the direct fluorination of polymeric resins and, more particularly, relates to the direct fluorination of polymeric resins in particulate or product form which are capable of being fluorinated by contact with elemental fluorine and a dense carrier fluid.
The fluorination of polymers to enhance lubricity, nonflammability and chemical inertness, such as resistance to oxidation, and to reduce permeability by organic solvents, particularly non polar solvents, by the replacement of hydrogen by fluorine by the use of fluorine itself or a metal fluoride is well known.
The July, 1962 publication of Chemical & Engineering News discusses in the article "New and Varied Paths for Fluorine Chemistry" the chemical reactions which occur when fluorine reacts with organic molecules. The Encyclopedia of Chemical Technology, Third Edition, (Volume 10) published by John Wiley & Sons discusses direct fluorination of organic compounds to produce fluorocarbon polymers by the direct reaction of fluorine with polyethylene.
The Polymer Letters Edition, Volume 12, (1974), contains the article "The Controlled Reaction of Hydrocarbon Polymers with Elemental Fluorine" which discloses the reaction of polyethylene and other finely powdered hydrocarbon polymers with fluorine.
The Journal of Polymer Science, Volume 13, pp 857-890, (1975), discloses in "Part VI Surface Fluorination of Polyethylene" the surface coating of polyethylene by replacement of hydrogen by fluorine.
U.S. Pat. No. 2,811,468 issued Oct. 29, 1957 discloses the fluorination of a surface of polyethylene film to render the surface substantially impermeable.
Fluorination of polymeric resins is usually carried out with carriers in the gas phase or in the liquid phase. In the liquid fluorination phase the temperature of operation is generally lower than room temperature and the fluorine concentration is lower than 2 mole per cent. In the gas phase fluorination, the temperature of operation is above the room temperature and the fluorine concentration generally is higher than 2 mole per cent.
Gaseous carrier fluids essentially are ideal gases with low heat capacity per unit of volume, low polarizability or ability to induce dipole moment, and low critical temperature as well as a cryogenic boiling point. When fluorine reacts with solid particles of polymeric resin, such as polyolefin resins including polyethylene, hydrogen in the polymer molecule combines with fluorine atoms to form hydrogen fluoride. The reaction is exothermic and, in order to prevent deterioration of the resin during fluorination due to local overheating, generated heat must be dissipated.
U.S. Pat. No. 4,536,266 issued Aug. 20, 1985 teaches a process for the fluorination of surface layers of articles by exposure of the surface layer to a fluorine-yielding liquid solvent or solvent mixture in a controllable reaction by controlled introduction of fluorine gas. It is stated in column 1, lines 53-68, of this patent that problems inherent in known prior art processes are: "Treatment in the gaseous phase entails great safety hazards, since the reaction heat is considerable and can only be removed with great difficulty . . . changes in temperature and fluorine concentration must be kept within narrow limits. Even so, it is impossible to fluorinate surface layers with repeatable uniformity, on account of local overheating." This patent accordingly was directed to the use of liquid solvents in order to overcome problems considered inherent with the use of gaseous carrier fluids.
It is a principal object of the present invention to provide a process for the fluorination of polymeric resins quickly, consistently and safely by the use of elemental fluorine in a vapour phase with a dense carrier fluid which effectively functions as a heat sink.
It is another object of the invention to select and provide a dense carrier fluid which can be mixed with elemental fluorine to maintain a constant reaction temperature to control the aggressiveness of the elemental fluorine on the reaction with polymeric resins and to provide uniform treatment of the resins.